The present invention relates to layers for components, and a process for forming the layers, and more specifically, to fluorinated carbon filled layers useful as layers for components used in electrostatographic processes including image on image, contact electrostatic printing, digital and like processes. In a preferred embodiment, the layers are used in xerographic members such as bias charging members. In embodiments, there are selected fluorinated carbon filled layers which are useful as layers for bias charging rolls, belts, films, sheets and other components. In embodiments, the present invention allows for the preparation and manufacture of bias charging members with superior electrical and mechanical properties, including controlled and uniform conductivity in a desired resistivity range, and increased mechanical strength, durometer, tensile strength, elongation and toughness. Further, in embodiments, the layers exhibit excellent wear properties such as a decrease or elimination of "bleeding" contaminants to the surface of an imaging member.
Methods of charging have been developed using a direct charging member for charging the imaging member. For example, U.S. Pat. No. 5,017,965 to Hashimoto et al., discloses a charging member having a surface layer which comprises a polyurethane resin. Also, European Patent Application 0 606 907 A1, discloses a charging roller having an elastic layer comprising epichlorohydrin rubber, and a surface layer thereover comprising a fluorine containing bridged copolymer.
These and other known charging members are used for contact charging for charging a charge-receiving member (photoconductive member or imaging member) through steps of applying a voltage to the charging member and disposing the charging member being in contact with the charge-receiving member. Such bias charging members require a resistivity of the outer layer within a desired range. Specifically, materials with too low resistivities will cause shorting and/or unacceptably high current flow to the imaging member. Materials with too high resistivities will require unacceptably high voltages. These adverse affects can also cause the bias charging members to have nonuniform resistivity across the length of the contact member. Other problems include resistivity that is susceptible to changes in temperature, relative humidity, and running time.
Attempts at controlling the resistivity within the desired range have focused on controlling the resistivity range at the pre and post nip areas. These attempts have included adding ionic additives to the elastomer layers. European Patent Application 0 596 477 A2, discloses a charging member comprising at least an elastic layer comprising epichlorohydrin rubber and a surface layer disposed thereon, the surface layer comprising at least a semiconductive resin and an insulating metal oxide contained in the semiconductive resin.
While addition of ionic additives to layers may partially control the resistivity of the layers to some extent, there are problems associated with the use of ionic additives. In particular, undissolved particles frequently appear in the layer which causes an imperfection in the layer. This leads to a nonuniform resistivity, which in turn, leads to poor charging properties and poor mechanical strength. Furthermore, bubbles appear in the conductive elastomer, some of which can only be seen with the aid of a microscope, others of which are large enough to be observed with the naked eye. These bubbles provide the same kind of difficulty as the undissolved particles in the elastomer namely, poor or nonuniform electrical properties, poor mechanical properties such as durometer, tensile strength, elongation, a decrease in the modulus and a decrease in the toughness of the material. In addition, the ionic additives themselves are sensitive to changes in temperature, humidity, operating time and applied field. These sensitivities often limit the resistivity range. For example, the resistivity usually decreases by up to two orders of magnitude or more as the humidity increases from 20% to 80% relative humidity. This effect limits the operational or process latitude. Moreover, ion transfer can also occur in these systems. The transfer of ions will lead to contamination problems, which in turn, can reduce the life of the machine. Ion transfer also increases the resistivity of the elastomer member after repetitive use. This can limit the process and operational latitude and eventually, the ion-filled elastomer component will be unusable.
Conductive particulate fillers, such as carbons, have also been used in an attempt to control the resistivity. U.S. Pat. No. 5,112,708 to Okunuki et al., discloses a charging member comprising a surface layer formed of N-alkoxymethylated nylon which may be filled with fluorinated carbon. Generally, carbon additives control the resistivities and provide stable resistivities upon changes in temperature, relative humidity, running time, and leaching out causing contamination to imaging members. However, carbon particles disperse poorly in layers. Further, the required tolerance in the filler loading to achieve the required range of resistivity has been extremely narrow. This along with the large "batch to batch" variation leads to the need for extremely tight resistivity control. In addition, carbon filled layer surfaces have typically had very poor dielectric strength and sometimes significant resistivity dependence on applied fields. This leads to a compromise in the choice of centerline resistivity due to the variability in the electrical properties, which in turn, ultimately leads to a compromise in performance.
Fluorinated carbon has been added to elastomer layers in an attempt to control conductivity within the desired range, and without the negative consequences resulting from use of metal oxides and/or carbon additives. See U.S. patent application Ser. No. 08/672,803, filed Jun. 28, 1996, entitled "Bias Charging Member With Fluorinated Carbon Filled Fluoroelastomer Outer Layer." However, it has been found that when these fluorinated carbon filled fluoroelastomer layers are used as an overcoat for a bias charging member, significant bleeding or leaching contamination to the imaging member results when stored for long periods of time. The result is deposition of unknown residues on the surface of the imaging member. The residues lead to dark line defects in subsequent prints and copies. The residues do not appear to permanently damage the imaging member, but result in unacceptable prints or copies.
A bias charging member may be shipped and stored as a component of a customer replaceable unit. The charging member may then be loaded in intimate contact with an imaging member (e.g., photoreceptor) under spring tension. A bleeding or contamination issue can occur when the customer replaceable unit is stored for extended periods of time in situations that involve elevated temperatures and humidity, for example, storing in a warehouse, trailer, or the like. A "standard accelerated storage test" can be used to determine if any transfer of contaminants occurs under these severe conditions. The storage test involves subjecting the components to one week of 50.degree. C. temperature and 95 percent relative humidity to see if any transfer of contaminants occurs under these severe conditions. A positive result here would indicate the possibility of contamination under more "normal storage" conditions of many months at less severe temperatures and humidity.
Therefore, there exists a specific need accomplished with the present invention, in embodiments thereof, for an outer surface layer for charging members which allows for a stable conductivity in the desired resistivity range without the problems associated with ionic additives and carbon additives, and without bleeding or leaching of contaminants to an imaging member resulting in print quality defects.